The present invention relates generally to the design of electronic circuit assemblies which may be subjected to compression and/or bending stresses transmitted through their enclosures, the encapsulating of such circuit assemblies, and the component (integrated circuit and other fragile components) layout and arrangement. More specifically the present invention relates to the design of a PCMCIA card and the use of a flexible potting material for encapsulating the internal PCMCIA circuit assembly. The selected potting material maintains its resilient and flexible characteristics over a wide temperature range. The acronym PCMCIA stands for Personal Computer Memory Card International Association. A related feature of the present invention is the manner in which the integrated circuits and other components are arranged in the PCMCIA card in order to reduce the risk of damage due to the unique environment and stresses to which PCMCIA cards are exposed. A further feature of the present invention is the use of a thinner base board or flexible polymer based media in order to reduce the force which the board transmits to the electronic components which are mounted on the board.
Printed circuit boards are typically constructed from a glass laminate board which is clad with a conductive layer for the creation of a printed circuit. By means of a photoresist coating and photographic art work, a circuit layout is created. The portions of the conductive layer which are not part of the designed circuit are etched off the glass laminate surface. The component leads may either be soldered directly to the surface of circuit pads or with larger components holes may be drilled through the circuit pads in order to receive the component leads. Flexible circuits are typically constructed on a polymer based film. The circuit traces and pad images are reverse imaged on a fine screen using photo resist and photographic artwork. These traces and pads are printed onto the polymer film using conductive silver ink or anisotropic adhesive and standard screen printing techniques, with different patterns on opposite sides of the film. Connections between sides are made by punching holes in the film and "printing" through the holes.
One type of component which has widespread use on circuit boards of the type disclosed herein is an integrated circuit or IC which includes a generally rectangular solid body with a series of leads exiting on opposite sides of the encapsulated IC body. These components are designed in such a way that their leads are to be soldered directly onto the surface of circuit pads which are provided as part of the circuit art work.
PCMCIA cards are widely used in computers and in computerized equipment for a number of functions, including data storage, data transfer, and for various I/O devices such as modems and GPS receivers. PCMCIA cards have a standardized package size and shape as well as a standardized interface. Most PCMCIA cards produced today enclose the circuit assembly in a thin walled metal case which provides limited protection for the internal assembly.
While most PCMCIA cards work reliably indoors, in passenger transport vehicles, and even in the field if the user is careful, PCMCIA cards tend to fail when they are stressed beyond the applicable PCMCIA specifications for shock, vibration, compression, twisting, and bending. Bending with both ends supported and a load applied near the center of the PCMCIA card appears to be particularly destructive. This specific type of loading can occur and frequently does occur when the user places the PCMCIA card in his or her rear pocket and then sits down. While careful handling of the PCMCIA cards would presumably preclude such load damage, PCMCIA cards are not always handled with the requisite care and thus the need for the present invention. The present invention significantly increases the reliability of PCMCIA cards in these types of environments where the PCMCIA cards are likely to be stressed beyond their design limits.
Typical failures to circuit assemblies when the design stress is exceeded include cracked or broken components, as well as cracked or broken solder joints or circuit traces. In view of the restricted space which is available for PCMCIA cards, most of these assemblies use relatively thin components which are surface mounted. The memory ICs which are used are typically in TSOP packages. These packages are typically plastic and measure approximately 17.78 mm (0.7 inches) in length, 7.62 mm (0.3 inches) to 12.7 mm (0.5 inches) in width, and 1.19 mm (0.047 inches) in thickness. There are a series of gull wing leads on opposite sides of the package. When a load is exerted on these ICs in a direction normal to the surface of the PCMCIA board, the IC package may crack or the solder joints of the leads to the circuit pads may break. If there is a load applied to the board beneath the IC package, such as bending the PCMCIA card along its length, this may force the board into a convex arc beneath the IC package which can either crack the package, break the solder connection of the leads to the circuit pads, or both.
It would be an improvement to the design of PCMCIA cards if their reliability could be increased or otherwise improved in these harsh environments. One approach to improve the reliability of such PCMCIA cards as provided by the present invention is to use a flexible potting material which encapsulates the components which are mounted onto the circuit board. Another improvement provided by the present invention is to orient the long axis of the components parallel with the width (short axis) of the case and transverse to the long axis of the case. A still further feature of the present invention is the use of a thinner glass laminate board or flexible printed circuit media in order to reduce the force which the board is able to transmit to a component mounted on the board when the board is flexed.
Although there have been a variety of PCMCIA circuit board and IC packaging concepts patented over the years, none of these earlier designs anticipate nor render obvious the combination of features of the present invention. The following listed patents are believed to provide a representative sampling of these earlier PCMCIA circuit board and IC packaging concepts:
______________________________________ PATENT NO. PATENTEE ISSUE DATE ______________________________________ 5,067,008 Yanaka et al. Nov. 19, 1991 5,079,673 Kodai et al. Jan. 7, 1992 5,173,840 Kodai et al. Dec. 22, 1992 5,244,840 Kodai et al. Sep. 14, 1993 5,515,595 Kurz May 14, 1996 5,525,644 Artus et al. Jun. 11, 1996 5,548,485 Bethurum et al. Aug. 20, 1996 5,554,821 Patterson et al. Sep. 10, 1996 ______________________________________
Based upon the disclosures of the listed patents, the packaging of ICs and the layout and arrangement of PCMCIA circuit boards has received a substantial degree of design attention over the years. However, the improvements offered by the present invention for the specific purpose of improving PCMCIA card reliability have not heretofore been the focus of any earlier inventions. The phrase "environmentally hardened" can be used to describe the PCMCIA card of the present invention in that the PCMCIA card is modified or enhanced in some way to be able to survive in a harsh environment.